In an electronic system, there is often a need to provide a flow of current into a load such that the current is constant almost regardless of an impedance of the load. For example, to set an amplifier at a particular operating point, a bias current can be applied. The bias current has to be maintained at a constant level. In one example, a constant bias current is needed for the control of oscillators and timing signals. In another example, a constant bias current is needed is for generation of high-voltage control signals used in a non-volatile memory, for example a flash memory.
A current source that can provide a constant current typically includes transistors that provide a current gain. Usually, the current gain varies substantially across instances of transistors. Therefore the current source needs to be designed such that an output current of the current source is independent of the current gain of the transistors. A current mirror is used to produce the constant current of the desired level.
Currently, electronics need to consume less current so as to maximize battery life. Hence, current sources need to supply reliable and low levels of the current. By Ohm's law, a low current can be produced by the current source by increasing a size of a resistor that determines a level of current. However, increasing the size of the resistor consumes more area on an integrated circuit. Also, a certain type of resistor is more likely to be subject to variation of resistance with temperature, thereby compromising a constancy of the current that is provided by the current source.
Hence, there is a need for a current source that consumes less power, occupies less area in an integrated circuit, and is stable against temperature fluctuations and other environmental factors, for example power supply voltage fluctuations and manufacturing variations.